In a gas turbine aircraft engine air enters at the engine inlet and flows into the compressor, which compresses the air. Compressed air flows to the combustor where it is mixed with injected fuel and the fuel-air mixture is ignited. The hot combustion gases flow through the turbine. The turbine extracts energy from the hot gases, converting it to power to drive the compressor and any mechanical load connected to the drive.
It is well known to use labyrinth seals in a gas turbine engine to control the flow of gases therethrough. For example, U.S. Pat. No. 3,589,475 to Alford discloses labyrinth seals arranged between rotating and stationary members. Such seals generally comprise a plurality of axially spaced circumferential teeth which extend into sealing relationship with a sealing surface. The seals of Alford are operative to minimize high-pressure fluid leakage from an annular passage into a chamber defined in part by a shaft member and the engine casing.
Although Alford recognized that there would be some leakage of high-pressure fluid from the annular passage through the clearance passages existing between the tip of each tooth and its respective sealing surface and into the chamber, Alford does not discuss the need in some cases to divert the fluid flow upon its exit from the labyrinth seal.
U.S. Pat. No. 4,190,397 to Schilling et al. shows a labyrinth seal arranged immediately aft of the compressor in a gas turbine engine. The turbine portion of the gas turbine engine is typically cooled by air pressurized by the compressor. This coolant air is bled from the compressor flow path through an annular gap between the last stage of compressor rotor blades and the outlet guide vanes and thereafter flows along a frustoconical rotor member into an annular passage in which the labyrinth seal is arranged. The coolant flow rate is metered by the labyrinth seal.
In order to obtain the desired metered amount of coolant flow and yet minimize overall engine performance degradation, the labyrinth seal of Schilling is designed to operate with minimal running clearances between the labyrinth seal teeth and the stationary honeycomb seal material. This minimal clearance causes a temperature rise in the air passing through the seal so that the air exiting the seal loses some of its useful cooling capacity. In addition, where the flanges aft of the labyrinth seal are bolted together, the bolt head and nuts produce turbulent mixing and churning of the coolant flow as it passes over them, creating aerodynamic drag between rotating and static parts and further raising the temperature of the coolant.
In an attempt to minimize any such additional temperature rise caused by such protrusions, Schilling employs an improved windage shield which reduces windage and turbulence. The improved windage shield comprises a continuous ring provided with a plurality of circumferentially spaced recesses having a contour similar to that of the bolt heads. The thickness of the bolt heads and the depth of the recesses are such that the bolt heads form a generally flush interface with the surrounding surfaces of the windage shield, thereby eliminating open access holes and upstream-protruding bolt heads.
Although the Schilling patent recognizes the need to control a property, i.e., the level of aerodynamic drag, of the air flow upon its exit from the labyrinth seal, Schilling does not anticipate the need in some instances to change the direction of that air flow.